1. Field of the Invention (Technical Field)
The invention relates to a counting device for the remote counting of stacked objects in a stack of thin objects such as carton sheets, comprising a radiation source for the generation of a radiation beam, means for moving the radiation beam along the transverse edges of the stacked objects, photoelectric detection means for the detection of the intensity differences in radiation reflected by the irradiated stacked objects, as well as optical means placed between said stacked objects and said photoelectric detection means.
2. Background Art
A counting device of the above-mentioned kind for courting a stack of corrugated board is known from the Dutch patent specification No. 167,530 as well as from the U.S. patent specification 3,581,067, referred to therein.
The latter patent specification reports that during the vertical scanning of the stack the charges in reflection behaviour occurring between the separate layers are counted. It is the changes in brightness caused by the edge characteristics of the layers of material which follow in quick succession, that are counted, while slow changes in the mean brightness of the material are ignored. The faster horizontal scanning periodically applied serves to determine the mean brightness over a larger horizontal width. To count the corrugated boards, the scanning light beams are directed onto the end face of the stack at an angle in order to obtain a mean brightness value which is independent of whether or not the light beam happens to fall on the front of a carton. The disadvantage of this method is that a slight change in brightness between the adjacent layers will cause faulty counting. The reason for there being so little change in brightness between adjacent layers may be that the material edges have generally poor reflection properties, or that the gap between the in themselves evenly reflecting layers, is too narrow. When counting the corregated boards by the known method, it is also possible that a split between two base sheets is counted as a layer.
The counting device described in the above-mentioned Dutch patent specification 167,530 uses a method in which a predetermined width of one end of the stack is at the same time scanned in a faster horizontal movement by means of a photoelectric sensor consisting of a horizontal group of photodiodes being switched through electric impulses alternately from one to the other end of the sensor, the impulses obtained are identified as coming from a flat base place when a series of connected impulses are detected, or from a corrugated plate of the corrugated board when a series of impulse groups is detected, or as coming from a split between adjacent corrugated boards when no signal is detected, and by means of a counting and reading device which is set at a base plate or zero detection so that a sheet of carton is counted if a certain number of impulse groups coming from the corrugated board is detected. The device used to carry out the above method is characterized in that elements represent for the provision of a narrow illuminated scanning zone of predetermined length, a group of photodiodes absorbing light reflected from the scanning zone and elements for successive excitation of the photodiodes, a base plate detector comprising a counter emitting a signal when a predetermined number of impulses generated by the photodiodes is counted, and in addition that a corrugated board detector is provided. The group of photodiodes may comprise for instance 64 photodiodes.
Another possibility applied in the known art involves the observance of the entire stack by means of a CCD camera, with a subsequent analyzation of the picture content.
The disadvantage of the method using the moving sensor is the complex construction of the mechanical part of the device, while the disadvantage of the method using a CCD camera is the relatively high cost of the CCD camera and the processing unit as well as the poor resolution.
It will be clear from the above that counting stacks of carton or stacks of other thin objects is technically not simple. The main reason for this is the relatively slight contrast between the reflection from the core and the reflection from the sides of the stacked objects when the counting device is not just used for counting sheets of corrugated board, but in particular also for solid carton, sheets of plastic foil and the like, where there are few or no openings in and/or between the sheets. In practice, the great variations in distance which often exist between the counting device and the stacks of objects to be counted also play a role because of the variations in sheet size with the stacks being moved on an aligned path along a fixed device. To avoid constantly having to move the counting device or the stacks of carton it is desirable to provide a counting device with a great focal depth. The deficient focal depth of the device of the known art has resulted in an increasingly complex counting device and in the addition of extra movements which increase the mechanic complexity of the device.